The present invention relates to a digital communication system and, more particularly, to a variable communication system which switches communication schemes in accordance with the communication amount.
In digital communication systems, there have been various proposals to efficiently transmit data. For example, in the digital communication system disclosed in Japanese Patent Laid-Open No. 7-123039, the traffic of communication is detected from an m-sequence signal input to a rate conversion multiplexer (not shown), and the rate conversion multiplexer multiplexes data at a rate corresponding to the traffic. A variable rate modulator (not shown) then modulates the multiplexed signal and transmits it to a transmission path at a transmission rate corresponding to the magnitude of the traffic. That is, the modulation clock rate is time-modulated. On the receiving side, a variable rate demodulator (not shown) performs demodulation, and a rate conversion demultiplexer (not shown) demultiplexes and rate-converts the demodulated signal to reproduce the data. Claim 2 in Japanese Patent Laid-Open No. 2000-316035 proposes a technique of efficiently assigning bands by dynamically changing the transmission rate in accordance with the transmission amount as in the above prior art.
In this manner, in the field of digital communication systems, when the communication amount is small, the transmission rate of information to be transmitted is set low to set a line with a low modulation clock rate, whereas as the communication amount increases, a line with a high modulation clock rate and high transmission rate is set. For example, a router which has selected a general voice line connects to a predetermined content server on the Internet at a given time point to transfer large-volume data. Likewise, when a cell phone switches to the videophone mode during voice communication, the communication amount increases at this time point. In a case opposite to the above case, the communication amount decreases at a predetermined time point.
Properly switching the transmission rates of information in accordance with the communication amount in this manner makes it possible to perform efficient communication. In general, however, when the modulation scheme to be used remains the same, if the transmission rate is low and the modulation clock rate is low, a line tends to relatively deteriorate because it is susceptible to the influences of phase noise in a local oscillator in an up converter which performs frequency conversion to obtain carrier waves or a down converter in a reception unit. This makes it impossible to obtain a stable transmission path. In general, therefore, it is preferable to use a line with a high transmission rate and high modulation clock rate.
In contrast, it is conceivable to use a technique of performing multilevel modulation in the same band by different modulation schemes while the modulation clock rate is kept constant. 16-QAM (Quadrature Amplitude Modulation) which can obtain 16 signal states is capable of obtaining a quadruple transmission rate at the same modulation clock rate as compared with BPSK (Bi-Phase Shift Keying) which is a modulation scheme of making bit information of a baseband correspond to two phases. When, however, multilevel modulation is to be performed, in order to keep transmission path quality (i.e., a bit error rate) equivalent to a modulation scheme with a low transmission rate, it is necessary to increase the output level by an amount equal to or more than a band improvement in terms of transmission rate. In the above case, although 16-QAM realizes a band improvement of 6 dB in terms of transmission rate as compared with BPSK, the transmission carrier power must be increased by 10 dB or more to obtain the same bit error rate at 10E-6 as that in BPSK.
In addition, when the transmission rates are switched in accordance with the communication amount, demodulation synchronization must be established with respect to the transmission rate after switching at the time of this switching operation. Demodulated data is temporarily interrupted until demodulation synchronization is established. Compare a case wherein the transmission rate after switching is high with a case wherein the transmission rare after switching is low. In the latter case, for example, in a carrier wave reproducing circuit, the frequency error of a reception carrier wave with respect to the modulation clock rate becomes relatively large. For this reason, in a PLL circuit for carrier wave reproduction, it takes a longer time to establish demodulation synchronization than in the case wherein the transmission rate after switching is high. Omission of communication data due to the interruption of demodulated data leads to a crucial result on the reproduction of information. In order to prevent omission of information due to the interruption of demodulated data, a buffer memory has been conventionally used. There has been a communication sequence in which when transmission or reception is to be performed, communication data is temporarily stored in the buffer memory, and transmission is stopped at a predetermined timing which poses no problem in the reproduction of information. In this sequence, when the receiving side detects omission of communication data, a re-transmission request is generated to read out the corresponding portion from the buffer memory on the transmitting side and re-transmit it.
In order to prevent omission of communication data and guarantee the perfection of reproduction of information by using such a technique, a considerably large buffer memory must be prepared. If information rates are set in two levels, i.e., a low rate and a high rate, as in the above case, in particular, the interruption of communication data becomes large in the case wherein the information rate is low. In this case, therefore, a buffer memory large enough to cover this must be prepared.